Multiple cavity tuning of a transmitter output in a communication system

ABSTRACT

A base-site (304) combines baseband frequency hopping and fast-synthesizer hopping to produce an economical frequency hopping communication system (300). The base-site (304) combines the fast-synthesizer frequency hopping capability of transmitters (307-309) with baseband frequency hopping to produce a frequency hopping communication system (300) which serves the same number of subscribers served by transmitters (208-213) in a purely baseband hopping communication system (200), but with fewer transmitters (307-309). The implementation of frequency-selective cavities (312-317) having very low loss eliminates the need for wideband hybrid combiners (112-114), which in turn eliminates transmitted-signal power loss experienced in a purely fast-synthesizer frequency hopping communication system (100).

This is a division of application Ser. No. 07/907,981, filed on Jul. 2,1992, now U.S. Pat. No. 5,263,047.

FIELD OF THE INVENTION

The invention relates generally to communication systems and morespecifically to frequency hopping cellular radiotelephone systems.

BACKGROUND OF THE INVENTION

In communication systems, and more specifically, cellular radiotelephonesystems, frequency hopping is used to increase subscriber capacity byreducing overall system interference. One such frequency hoppingtechnique, fast-synthesizer frequency hopping, is useful to realizesmall increases in subscriber capacity. Fast-synthesizer frequencyhopping allows a single transmitter to hop over an arbitrary number offrequencies. To provide multiple carriers in a cell (i.e., an increasein subscriber capacity), signals transmitted at a transmitter's outputare combined utilizing wideband hybrid combiners. Unfortunately, thesewideband hybrid combiners provide at least 3 dB of loss per stage ofcombining. Consequently, power loss of a signal transmitted by atransmitter increases with the number of channels provided.

Another frequency hopping technique used for high capacity applicationsis baseband frequency hopping. Baseband frequency hopping is useful torealize large increase in subscriber capacity. In baseband frequencyhopping, fixed-frequency transmitters (i.e., transmitters that do notfast-synthesizer frequency hop) are interconnected to antennas throughfrequency-selective cavities to achieve low-loss combining of signalstransmitted by the transmitters. Frequency hopping is achieved bydistributing baseband information to all the transmitters withappropriate synchronization. Unfortunately, the number of hoppingfrequencies is limited to the member of fixed-frequency transmittersemployed.

Thus, a need exists for an economical means to hop over a member offrequencies to yield a mid-range subscriber capacity increase, yet stillmitigate power loss of a signal transmitted.

SUMMARY OF THE INVENTION

A base-site in a communication system comprises a transmitter fortransmitting variable frequencies signals at an output, and at leastfirst and second cavities tuned to first and second frequenciesrespectively, the first and second cavities each having as input theoutput of the transmitter and having an output coupled to a commonantenna.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 generally depicts a base-site which implements fast-synthesizerfrequency hopping.

FIG. 2 generally depicts a base-site which implements baseband frequencyhopping.

FIG. 3 generally depicts a base-site which implements economicalfrequency hopping for mid-range capacity which mitigates transmitterpower loss in accordance with the invention.

FIG. 4 generally depicts how transmission of frequencies F1-F6 aredistributed for three subscribers in accordance with the invention.

FIG. 5 generally depicts how frequencies F1-F6 and a dedicated broadcastcontrol channel (BCCH) frequency are distributed for three subscribersin accordance with the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 generally depicts a communications system 100 implementing abase-site 104 which performs fast-synthesizer frequency hopping. Asdepicted in FIG. 1, a Mobile Services switching Center (MSC 102),couples base-site 104 to a Public Switched Telephone Network (PSTN). MSC102 is coupled to a plurality of transmitters 108-111 via atime-division multiplexed (TDM) bus 106. TDM bus 106 is well understoodin the art, and may be of the type described in U.S. Pat. No. 5,081,641,having as inventors Kotzin et al. Continuing, transmitters 108-111 aretime-division multiple access (TDMA) transmitters, having performancecharacteristics described in GSM Recommendation 5.05, Version 3.11.0,published March, 1990. Output from transmitters 108-111 are input into 3dB wideband hybrid combiners 112-114 which provide wideband frequencycombining of signals transmitted by transmitters 108-111. In thepreferred embodiment, transmitters 108-111 transmit signals havingfrequencies in the range of 935 MHz to 960 MHz. Consequently, 3 dBhybrid combiners 112-114 are able to combine signals having frequencieswithin that range. Combined signals exiting 3 dB hybrid combiner 114 areeventually transmitted to subscribers (not shown) via a common antenna115.

During fast-synthesizer frequency hopping, TDM bus 106 provides a mediumin which packetized information (within timeslots of the TDMA system)are distributed from MSC 102 to transmitters 108-111 for transmission.Transmitters 108-111 receive the packetized information, and transmit ata predetermined frequency during a particular timeslot. In a subsequenttimeslot, a particular transmitter, for example transmitter 108 wouldtransmit at one frequency to a subscriber during a timeslot thentransmit at a different frequency, to either the same or a differentsubscriber, during a subsequent timeslot. As is clear to one of ordinaryskill in the art, transmitters 108-111 continuously change frequency, orfrequency hop, from timeslot-to-timeslot.

Base-site 104 of FIG. 1 provides adequate performance when smallsubscriber capacity increases are required. For purposes of example,base-site 104 of FIG. 1 is depicted as prodding three different carriers(frequencies) of capacity (in addition to a broadcast channel (BCCH)carrier frequency which does not frequency hop). As is obvious to one ofordinary skill in the art, this configuration would work over any numberof different carriers. In this configuration, however, each frequencytransmitted by transmitters 109-111 will experience at least 6 dB ofloss due to combiners 112-114. Since transmitters 109-111 experiencesuch a severe loss in transmitter power, the physical size of the cell,or coverage area, to which transmitters 109-111 serve is effectivelydecreased. This in turn minimizes capacity per coverage area, whichessentially offsets the capacity gains realized by the fast-synthesizerfrequency hopping configuration depicted in FIG. 1.

FIG. 2 generally depicts a base-site 204 which implements basebandfrequency hopping. As depicted in FIG. 2, MSC 102 and TDM bus 106 may besimilar as those depicted in FIG. 1. Also, coupled to TDM bus 106 aretransmitters 207-213. In keeping consistent with the example describedabove for FIG. 1, base-site 204 is required to hop over at least sixfrequencies. Therefore, six transmitters 208-213, (excluding the BCCHtransmitter 207) and six frequency responsive means, which in thepreferred embodiment are frequency-selective cavities 216-221 (excludingfrequency-selective cavity 215 for the BCCH frequency), are required.Frequency selected cavities 216-221 are tuned to predeterminedfrequencies F₁ -F₆ before installation into a cell-site. Consequently,transmitters 208-213 can only transmit signals having frequencies F₁ -F₆respectively. If transmitters 208-213 transmit any other frequency otherthan F₁ -F₆ respectively, frequency-selective cavities 216-221 willreject those signals.

To implement baseband frequency hopping with base-site 204, packetizedinformation sent from MSC 102 to TDM bus 106 is synchronized and routedto the appropriate transmitter 208-213 for transmission via commonantenna 115. Transmitters 208-213, during baseband frequency hopping, donot change frequencies; each transmitter 208-213 is fixed to thepredetermined frequency F₁ -F₆ to which frequency-selective cavities216-221 are tuned. Baseband frequency hopping occurs when the packetizedinformation which is routed to different transmitters 208-213 containsinformation for a single subscriber. For example, baseband informationreceived from a particular transmitter, say transmitter 208, may beintended for a particular subscriber during a particular timeslot. In asubsequent timeslot, packetized information received by a differenttransmitter, say transmitter 209, may be intended for the samesubscriber. As this process continues, transmitters 208-213, while eachstaying on a fixed frequency, take turns (during successive timeslots)transmitting information to a particular subscriber. Obviously, ahopping pattern may be determined so that many more than one subscriberat a time is served. However, the use of six separate transmitters208-213 to perform baseband frequency hopping is an extremely costlysolution to frequency hopping over only six frequencies.

FIG. 3 generally depicts a base-site 304 which implements economicalfrequency hopping for mid-range subscriber capacity increase whichmitigates the power loss of a signal transmitted in accordance with theinvention. As depicted in FIG. 3, MSC 102 and TDM bus 106 are similar tothose depicted in FIGS. 1 and 2. Also depicted in FIG. 3 aretransmitters 306-309 and frequency-selective cavities 311-317.Transmitters 306-309 are each capable of transmitting variable frequencysignals at their output. As can be seen, only three transmitters 307-309are used in conjunction with six frequency-selective cavities 312-317 toprovide frequency hopping over six different frequencies. Obviously, byincorporating more frequency-selective cavities per transmitter, morefrequencies can be hopped over. Each transmitter 307-309 has its outputinput to a set of frequency-selective cavities 312-313, 314-315, and316-317 respectively. Each frequency-selective cavity in the set offrequency-selective cavities are tuned to first and second predeterminedfrequencies and have an output coupled to common antenna 115. To providefrequency hopping in accordance with the invention, each transmitter307-309 transmits a signal at a predetermined frequency such that nocommon predetermined frequencies are transmitted via common antenna 115at the same time.

Frequency hopping in the communication system 300 of FIG. 3 is performedas follows. Each transmitter 307-309 is capable of fast-synthesizerfrequency hopping utilizing a synthesizer which is capable of generatingthe required output frequency from timeslot-to-timeslot. For example,transmitter 307 is capable of switching frequencies between F₁ and F₂from timeslot-to-timeslot. When packetized information is sent totransmitter 307, transmitter 307 transmits at either F₁ or F₂, but neverboth simultaneously. This same process occurs for transmitters 308, 309,and however many other transmitters may be employed in the communicationsystem 300). In a subsequent timeslot, packetized information enterstransmitter 307 and a message within the packetized informationinstructs transmitter 307 to transmit a signal at F₂ during thatsubsequent timeslot. Depending on the hopping pattern, transmitter 307may hop between frequencies F₁ and F₂, or may simply stay tuned to F₁and F₂ depending on the frequency hopping requirements of communicationsystem 300.

FIG. 4 generally depicts how transmission of frequency F₁ -F₆ aredistributed for transmission to three subscribers in accordance with theinvention. In the preferred embodiment, each transmitter 307-309transmits a carrier which comprises 8 timeslots to serve up to eightdifferent subscribers. Obviously, more than 24 subscribers (8timeslots×3 transmitters in this example) may be served at any one timeby communication system 300, with the only requirement that moretransmitters be added to the system; however, there is no real limit tothe number of frequency-selective cavities that may be coupled to theoutput of a particular transmitter 307-309. The advantage ofcommunication system 300 depicted in FIG. 3 is that an increase incapacity can be realized over communication system 100 of FIG. 1 whilemaintaining the required/desired power level output from transmitters307-309. Frequency-selective cavities 312-317 are low-loss cavitieswhich do not present a 3 dB loss to a signal that is transmitted. Sincefrequency-selective cavities are low-loss, base-site 304 of FIG. 3 mayincorporate transmitters 307-309 coupled to more than twofrequency-selective cavities in a particular set of frequency-selectivecavities.

Returning to FIG. 4, there is depicted one of many hopping sequenceswhich may be used to serve, for example three subscribers, with afrequency-hopped transmission in accordance with the invention. Asdepicted in FIG. 4, subscribers 1, 2, and 3 are shown having a series oftransmissions F₁ -F₆ as seen by subscribers 1, 2, and 3. For example, ina synchronized timeslot (a timeslot in which each subscriber 1, 2, and 3would each see a common transmission) such as timeslot 401, subscriber 1would be served by transmitter 307 via frequency-selective cavity 312 ata frequency of F₁. During the same timeslot (not physically the sametimeslot; same in that they are synchronized), the second subscriber,subscriber 2, would be served by transmitter 308 via frequency-selectivecavity 313 at a frequency F₃. Likewise, subscriber 3 would be served bytransmitter 309 via frequency-selective cavity 316 at a frequency F₅. Ina subsequent timeslot 402, transmitters 307-309 would fast-synthesizerfrequency hop to the other predetermined frequency in the set of set ofpredetermined frequencies (for example, transmitter 307 would frequencyhop to F₂ out of the set of F₁ and F₂). Consequently, during timeslot402, subscriber 1 would be served by transmitter 307 viafrequency-selective cavity 313 at a frequency F₂, subscriber 2 served bytransmitter 308 via frequency-selective cavity 313 at a frequency F₄,and subscriber 3 served by transmitter 309 via frequency-selectivecavity 317 at a frequency F₆. During the transition from timeslot 402 totimeslot 403, base-site 304 would baseband frequency hop such that eachtransmitter 307-309 would serve a different subscriber than the previoustwo timeslots. For example, during timeslot 403, subscriber 1 would beserved by transmitter 308 via frequency-selective cavity 314 and afrequency F₃. Likewise, subscriber 2 would be served by transmitter 309via frequency-selective cavity 316 at a frequency F₅, and subscriber 3would now be served by transmitter 307 via frequency-selective cavity312 at a frequency F₁.

Significant to note is that FIG. 4 illustrates the necessity for thebaseband information distribution capability. That is, a subscribersinformation is required at all transmitters. Also note that at no timeis F₁ on with F₂, F₃ with F₄, or F₅ with F₆. This is necessary sinceonly one transmitter is provided for the set of predeterminedfrequencies.

As depicted in FIG. 3, the BCCH transmitter 306, and the correspondingBCCH frequency-selective cavity 311 are also coupled to common antenna315. Typically, in TDMA communications systems, such as communicationsystem 300, transmission by BCCH transmitter 306 must occur during adedicated timeslot. In the preferred embodiment of the GSM digitalradiotelephone system, the dedicated timeslot is timeslot zero of a8-timeslot frame. For more information on the framing structure of thepreferred embodiment, reference is made to GSM Recommendation 5.01,Version 3.3.1, published January, 1990. Continuing, in keepingconsistent with the above example of hopping over six frequencies, FIG.5 depicts transmission of a signal at the BCCH frequency, denoted byF_(b) in FIG. 5, every 7 timeslots. As can be seen, during timeslot 501,subscriber 1 receives the BCCH signal transmitted by BCCH transmitter306 via frequency-selective cavity 311 at F_(BCCH). Subscriber 2 isserved by transmitter 307 via frequency-selective cavity 313 at afrequency F₂, while subscriber 3 is served by transmitter 308 viafrequency-selective cavity 315 at frequency F₄. As is apparent to one ofordinary skill in the art, subsequent timeslots 502, 503, and othersserve subscribers 1, 2 and 3 via the combination of baseband frequencyhopping and fast-synthesizer frequency hopping as described above.Again, significant to note is that the paring rule described above(between each frequency-selective cavity in the set offrequency-selective cavities) is never violated. In this manner, theelements of baseband frequency hopping and fast-synthesizer frequencyhopping are combined to provide a economical (the number of transmitterscut in half) solution to frequency hopping without affecting the powerlevel (absence of 3 dB hybrid combiners) of a signal transmitted bytransmitters 307-309.

Thus, it will be apparent to one skilled in the art that there has beenprovided in accordance with the invention, a method and apparatus forproviding multiple cavity tuning of a transmitter output in acommunication system that fully satisfies the objects, aims, andadvantages set forth above.

While the invention has been described in conjunction with specificembodiments thereof, it is evident that many alterations, modifications,and variations will be apparent to those skilled in the art in light ofthe foregoing description. Accordingly, it is intended to embrace allsuch alterations, modifications, and variations in the appended claims.

What we claim is:
 1. A base-site in a communication system, comprising:atransmitter for transmitting variable frequency signals at an output;and at least first and second cavities tuned to first and secondfrequencies respectively, the first and second cavities each having asinput the output of the transmitter and having an output coupled to acommon antenna.
 2. The base-site of claim 1 wherein the transmitter fortransmitting variable frequency signals further comprises a transmitterfor transmitting signals at the first and second frequencies of thefixed-frequency cavities.
 3. The base-site of claim 2 wherein thetransmitter for transmitting signals at the first and second frequenciesof the fixed-frequency cavities further comprises a transmitter fortransmitting a signal at either the first or the second frequency at atime.
 4. A transmission system comprising:a transmitter for transmittingvariable frequency signals at an output; and first and second cavities,coupled to the output of said transmitter and tuned to first and secondfrequencies respectively, said first and second cavaties each having anoutput coupled to a common antenna.
 5. The transmission system of claim4 wherein the transmitter for transmitting variable frequency signalsfurther comprises a transmitter for transmitting signals at the firstand second frequencies of the tuned cavity.
 6. The transmission systemof claim 5 wherein the transmitter for transmitting signals at the firstand second frequencies of the tuned cavity further comprises atransmitter for transmitting a signal at either the first or the secondfrequency at a time.
 7. A base-site in a communication system, thebase-site transmitting signals at first and second frequencies, thebase-site comprising:a transmitter for transmitting variable frequencysignals at an output; and first and second frequency responsive means,coupled to the output of said transmitter, for frequency-tuning signalstransmitted at said first and second frequencies, said first and secondfrequency responsive means each having an output coupled to a commonantenna.
 8. The base-site of claim 7 wherein said transmitter transmitssaid variable frequency signals such that said signals transmitted atsaid first and second frequencies are not simultaneously transmitted viathe common antenna.
 9. The base-site of claim 7 wherein said base-siteis a time division multiple access (TDMA) base-site.
 10. The base-siteof claim 9 wherein said transmitter for transmitting variable frequencysignals at an output further comprises a transmitter for transmittingvariable frequency signals at an output during a TDMA timeslot.
 11. Thebase-site of claim 8 wherein said transmitter for transmitting variablefrequency signals at an output during a TDMA timeslot further comprisesa transmitter for transmitting signals at first and second frequenciessuch that said signals are not simultaneously transmitted during thesame TDMA timeslot.
 12. A method of transmission in a communicationsystem, the method comprising the steps of:transmitting, via atransmitter, variable frequency signals; and frequency-tuning, via firstand second frequency responsive means having outputs coupled to a commonantenna, signals transmitted at first and second frequencies fortransmission.
 13. The method of claim 12 wherein said step offrequency-tuning signals transmitted at first and second frequencies fortransmission further comprises the step of frequency-tuning signalstransmitted at first and second frequencies for transmission via thecommon antenna.
 14. The method of claim 12 wherein said step oftransmitting variable frequency signals further comprises transmittingsignals at first and second frequencies such that said signals are notsimultaneously transmitted.
 15. The method of claim 12 wherein saidcommunication system is a time division multiple access (TDMA)communication system.
 16. The method of claim 15 wherein said step oftransmitting variable frequency signals further comprises the step oftransmitting variable frequency signals during a TDMA timeslot.
 17. Themethod of claim 16 wherein said step of transmitting variable frequencysignals during a TDMA timeslot further comprises the step oftransmitting signals at first and second frequencies such that saidsignals are not simultaneously transmitted during the same TDMAtimeslot.